Diamondback Moth (Lepidoptera: Plutellidae) Population Density and Parasitism by Diadegma insulare on Collard in South Carolina 1

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1 Diamondback Moth (Lepidoptera: Plutellidae) Population Density and Parasitism by Diadegma insulare on Collard in South Carolina 1 Mohamed F. R. Khan, Randall P. Griffin, 2 Gerald R. Carner, 2 and Clyde S. Gorsuch 2 Soil Science Department, North Dakota State University, 227 Walster Hall, Fargo, North Dakota USA J. Agric. Urban Entomol. 21(3): (July 2004) ABSTRACT The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is the most important insect pest of collard, Brassica oleracea var. acephala de Condolle, in South Carolina. In 1997 and 1998, studies were conducted on diamondback moth larval and pupal population density and parasitism by Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae), in collard fields in South Carolina. The density of diamondback moth larvae generally increased from below the economic threshold early in collard crop growth to above the economic threshold as crop growth progressed. Host density ranged from 0 to 9.28 diamondback moth larvae and pupae per plant. Data suggest that all sites would have required additional strategies to control the diamondback moth populations. Diadegma insulare was the major parasitoid of diamondback moth, and parasitism ranged from 0 to 58.5%. Diamondback moth was also parasitized by Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae). Parasitism by O. sokolowskii started later in the season than parasitism by D. insulare, usually when the diamondback moth population was at or above the economic threshold. Parasitism by O. sokolowskii ranged from 0 to 7.3%. Parasitism by D. insulare and O. sokolowskii was inadequate to maintain diamondback moth populations below the economic threshold in commercial collard fields in South Carolina. KEY WORDS Diadegma insulare, Plutella xylostella, collard, diamondback moth, population density, parasitism Collard, Brassica oleracea var. acephala de Condolle, is the most important crucifer cultivated in South Carolina. Lexington County cultivates the highest acreage and contributes $2.1 million of the collard crop conservatively valued at $3.3 million (South Carolina Agricultural Statistics Service 1996). South Carolina produces collard for the fresh markets in South Carolina, North Carolina, Florida, Virginia, Tennessee, and Pennsylvania. Fresh-market collards have a high cosmetic standard and provide for one grade U.S No. 1. The standard requires that collard selected for the fresh market must be insect free. 1 Accepted for publication 31 March Department of Entomology, Soils, and Plant Sciences. Clemson University, 114 Long Hall, Box , Clemson, South Carolina

2 KHAN et al: Diamondback Moth Density and Parasitism by Diadegma insulare 165 The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is the most serious insect pest of Brassicaceae worldwide (Talekar & Shelton 1993), and the most important insect pest of collard in South Carolina. Inconsistent control of the diamondback moth with synthetic insecticides, and later with the biological insecticide, Bacillus thuringiensis, resulted in the need to develop a holistic approach for managing the diamondback moth on collard in Lexington County, South Carolina. Extension educators at Clemson University, South Carolina, decided that a strategy integrating cultural practices, scouting, using natural enemies, or improving the environment to enhance the effectiveness of natural enemies of the diamondback moth, and the judicious use of B. thuringiensis products and new chemistry insecticides would be developed for managing diamondback moth. All stages of the diamondback moth have numerous natural enemies, although larval parasitoids are most prevalent and effective at suppressing diamondback moth populations (Goodwin 1979, Hamilton 1979, Cock 1983, Sastrosiswojo & Sastrodihardjo 1986, Ooi & Lim 1989, Mustata 1992). Consequently, larval parasitoids have been more widely used to manage diamondback moth populations. Diadegma insulare Cresson (Hymenoptera: Ichneumonidae) is the most important parasitoid of diamondback moth in North America (Harcourt 1960, 1963), and South Carolina (Muckenfuss 1992, Muckenfuss & Shepard 1994). Diadegma insulare, an endoparasitoid, feeds within the diamondback moth larva and emerges from the prepupa shortly after the host forms its cocoon. The larva of D. insulare then spins its own cocoon within that of the host and pushes the remnants of the cadaver to the bottom of the host cocoon (Harcourt 1986). The synchronization of D. insulare with its host s developmental stage and excellent searching capacity makes it suitable for use as a supplemental method for the integrated management of diamondback moth (Harcourt 1969, 1986). Additional information was needed to develop an integrated management strategy for the diamondback moth. Data were needed to determine the potential contribution of D. insulare for controlling the diamondback moth. Our study was conducted to determine diamondback moth larval and pupal population density and percent parasitism by D. insulare in untreated collard field plots. Materials and Methods In 1997, studies were conducted in Lexington County, South Carolina, at the Clinton Sease Farm from 20 June to 5 August, the Clayton Rawl Farm from 29 August to 26 September, and the Walter Rawl and Sons Farms from 5 September to 10 October. In 1998, studies were conducted at the Clinton Sease Farm from 29 July to 2 September, the Clayton Rawl Farm from 16 July to 19 August, and the Walter Rawl and Sons Farm from 12 August to 14 September. At each farm, experimental sites, 22.5 m in width and 22.5 m in length were selected within commercial fields transplanted with 4-week old Top-Bunch collard seedlings. Plant spacing was 30 cm within rows that were 0.9 m in width and 22.5 m in length. Four plots, each 1.8 m (two rows) in width and 4.5 m in length, within each experimental site, were selected and demarcated by flags. Diamondback moth larvae and pupae were counted, but not removed, on five randomly selected plants from each of the selected four plots weekly until harvest. Host density was determined as the mean number of diamondback moth larvae and pupae per collard plant.

3 166 J. Agric. Urban Entomol. Vol. 21, No. 3 (2004) Fourth-instar diamondback moth larvae and pupae were collected weekly from plants within the experimental sites, other than from the four selected plots. Lower numbers (25) of diamondback moth larvae and pupae were collected during early crop growth, and higher numbers were collected during the latter stage of crop growth when the diamondback moth population increased. Larvae and pupae were collected in plastic cups in groups of approximately 25 and stored in a cooler with ice during collection and transportation. The diamondback moth larvae and pupae were taken to the laboratory and reared at 27 C and 70 ± 10% RH, with a photoperiod of 14:10 (L:D) h. Diamondback moth larvae were placed in cages and fed fresh collard leaves daily until pupation. Diamondback moth pupae were collected as cocoons attached to leaves. Leaf sections (3 3 cm) with one pupa each, were cut and placed individually into 30-ml plastic soufflé cups and kept in the rearing room. Cups were checked daily and adult diamondback moth and all parasitoid emergences were recorded. Diamondback moth larval and pupal mortality was also recorded. Voucher specimens of parasitoids were placed in the Clemson University Arthropod Collection, Department of Entomology, Clemson, South Carolina. Percent parasitism of diamondback moth by D. insulare was calculated using the formula [(number of D. insulare adults) / (number of adult insects (diamondback moth + D. insulare + others) that emerged)] 100. Results At the Clinton Sease Farm (Fig. 1A), diamondback moth density ranged from 0.15 to 1.45 larvae and pupae per plant at harvest. Three hundred and ninety-seven diamondback moth larvae and pupae were collected. Eighty-two (21%) diamondback moth larvae and pupae died in rearing, probably due to handling during collection and transportation to the laboratory, or to natural causes such as pathogens. Parasitism of diamondback moth by D. insulare ranged from 0% to 19%. At the Clayton Rawl Farm (Fig. 1B), diamondback moth density ranged from 0.15 to 2.25 larvae and pupae per plant at harvest. Three hundred seventy-five diamondback moth larvae and pupae were collected. Parasitism of diamondback moth by D. insulare ranged from 0% to 14.3%. At the Walter Rawl and Sons Farm (Fig. 1C), diamondback moth density ranged from 0.6 to 5.1 larvae and pupae per plant at harvest. Four hundred forty-one diamondback moth larvae and pupae were collected. Parasitism of diamondback moth by D. insulare ranged from 0% to 8%. The gregarious larvalpupal parasitoid, Oomyzus (= Tetrastichus) sokolowskii Kurdjumov (Hymenoptera: Eulophidae) was also obtained from diamondback moth collected on the last three collection dates. Parasitism of diamondback moth by O. sokolowskii was 3.5% At the Clinton Sease Farm (Fig. 1D), diamondback moth density ranged from 0.85 to 5.55 larvae and pupae per plant at harvest. Five hundred thirty diamondback moth larvae and pupae were collected. Parasitism of diamondback moth by D. insulare started early in the crop growth and ranged from 0 to 58.5%. Oomyzus sokolowskii was also present on the last two collection dates. Percent parasitism of diamondback moth by O. sokolowskii was 6.3% and 7.3% on 26 August and 2 September, respectively.

4 KHAN et al: Diamondback Moth Density and Parasitism by Diadegma insulare 167 Fig. 1. Mean densities of diamondback moth larvae and pupae (with SEM bars) and percent parasitism by Diadegma insulare. A, Clinton Sease Farm 1997; B, Clayton Rawl Farm 1997; C, Walter Rawl and Sons Farm 1997; D, Clinton Sease Farm 1998; E, Clayton Rawl Farm 1998; F, Walter Rawl and Sons Farm At the Clayton Rawl Farm (Fig. 1E), diamondback moth density ranged from 0.20 to 4.75 larvae and pupae per plant at harvest. Four hundred and twenty diamondback moth larvae and pupae were collected. Parasitism of diamondback moth by D. insulare ranged from 0 to 9.0%. Parasitism of diamondback moth by O. sokolowskii was 2.6% on 19 August. At the Walter Rawl and Sons Farm (Fig. 1F), diamondback moth density ranged from 0.05 to 9.25 larvae and pupae per plant at harvest. Four hundred

5 168 J. Agric. Urban Entomol. Vol. 21, No. 3 (2004) forty diamondback moth larvae and pupae were collected. Parasitism of diamondback moth by D. insulare ranged from 0% to 7%. Oomyzus sokolowskii was also present on the last three collection dates. Parasitism of diamondback moth by O. sokolowskii ranged from 2% to 4.6%. Discussion The economic threshold for diamondback moth on collard is one larva per plant (Muckenfuss & Shepard 1994). The host density of diamondback moth larvae and pupae generally increased from below the economic threshold early in collard crop growth to above the economic threshold as crop growth progressed. Host density ranged from 0.05 to 9.28 larvae and pupae per plant at harvest. The economic threshold was reached from 2 to 6 weeks after transplanting for collard that was ready for harvest about five to seven weeks after transplanting. All sites probably would have suffered economic losses, and required additional strategies to control the diamondback moth populations. Diamondback moth was parasitized mainly by D. insulare, and to a lesser extent, by O. sokolowskii. Parasitism by D. insulare ranged from 0% to 58.5%. At all sites, there was no parasitism of diamondback moth by D. insulare early in collard crop growth, probably because D. insulare was absent. Parasitism by D. insulare tended to increase as the diamondback moth population increased and as crop growth progressed. Except for the Clinton Sease Farm site study in 1997, parasitism by D. insulare started when the diamondback moth population density was at or above the economic threshold. Diamondback moth parasitism by O. sokolowskii ranged from 0% to 7.3%, and started later in the season than parasitization by D. insulare. The diamondback moth host density study was conducted within the site where the parasitism study was conducted. As such, parasitism rates probably would have been similar in the host-density study. Parasitism by D. insulare and O. sokolowskii was ineffective at keeping the diamondback moth population in the host-density study below the economic threshold. These results of low parasitism by D. insulare, and even lower percent parasitism by O. sokolowskii, are consistent with results obtained from similar studies by Alam (1992) on cabbage in Jamaica. The results are also consistent with Harcourt s (1963) observations that D. insulare by itself cannot effectively manage diamondback moth populations in crucifers grown on a commercial scale, particularly for the fresh market where cosmetic damage is greatly discounted. Harcourt (1963) claimed that D. insulare cannot provide full diamondback moth control except in combination with other appropriate parasitoid species. In Lexington County, the highest combined parasitism of 65.8% by D. insulare and O. sokolowskii was inadequate at maintaining the diamondback moth population below the economic threshold. Contrary to our results, Muckenfuss & Shepard (1994) reported more than 90% parasitism of diamondback moth by D. insulare in untreated collard plots in experiments during April through June in at the Coastal Research and Education Center (CREC), Charleston, South Carolina. At CREC, D. insulare and other natural enemies maintained the diamondback moth population below the economic threshold in untreated plots throughout the experiment, except for two weeks when the population reached less than 1.5 larvae per plant. Most pesticides have an adverse effect on population densities of D. insulare (Idris &

6 KHAN et al: Diamondback Moth Density and Parasitism by Diadegma insulare 169 Grafius 1993). The high rate of parasitism at CREC may have been the result of experiments being conducted in an area that was not as heavily treated with pesticides as the growers fields in Lexington County, where for many years, collard fields were treated with insecticides on a weekly basis. The CREC site also was probably more conducive for natural enemies because of a more diverse flora because it was not as intensively cropped as the Lexington County sites, where growers kept their fields and field margins weed-free. This study showed that diamondback moth populations can increase quickly over time to cause economic damage. This study also confirms that D. insulare is the major parasitoid of diamondback moth on collard in Lexington County, South Carolina. Natural parasitoid populations alone in commercial fields cannot control diamondback moth and provide high-quality collard demanded by the fresh market. Growers probably can enhance the population of parasitoids by providing suitable food sources in and around collard fields. Diadegma insulare lived longer and were more fecund when fed on wild mustard, Brassica kaber (D.C) Wheeler; yellow rocket, Barbarea vulgaris R. Br; and wild carrot, Daucus carota L. that produced a good source of nectar (Idris & Grafius 1997). Gourdine et al. (2003) suggested that kale, B. oleracea L. var. acephala, floral nectar and honey can increase the survival of D. insulare. Higher populations of high-fecundity, longer living D. insulare, complemented with adequate populations of introduced effective parasitoids and predators, may make a more significant pest-control contribution, especially early in collard growth when lower densities of diamondback moth larvae are present. Acknowledgments The authors thank Mr. Clayton Rawl, Mr. Chris Rawl, Mr. Howard Rawl, Mr. Charles Wingard, and Mr. Clinton Sease for allowing us to conduct research on their farms. The authors thank Clemson University s Extension Integrated Pest Management Program for funding this research. References Cited Alam, M. M Diamondback moth and its natural enemies in Jamaica and some other Caribbean countries, pp In N. S. Talekar [Ed.], Diamondback moth and other crucifer pests. Proceedings of the Second International Workshop, Tainan, Taiwan, December Taipei, Taiwan: Asian Vegetable Research and Development Center. Cock, M. J. W Introduction of parasites of Plutella xylostella (L) into Republic of Cape Verde Islands. Project Report 77.21AA /1300. Commonwealth Institute of Biological Control. Berkshire, United Kingdom, 12 pp. Goodwin, S Changes in the numbers in the parasitoid complex associated with diamondback moth Plutella xylostella (L) (Lepidoptera), in Victoria. Aust. J. Zool. 27: Gourdine, J. S., G. S. McCutcheon, A. M. Simmons & G. L. Leibee Kale floral nectar and honey as food sources for enhancing longevity and parasitism of Diadegma insulare (Hymenoptera: Ichneumonidae) a parasitoid of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae). J. Agric. Urban Entomol. 20: 1 6. Hamilton, J. T Seasonal abundance of Pieris rapae (L.), Plutella xylostella, (L.) and their diseases and parasites. Gen. Appl. Entomol. 11:

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